Generally, a wheel loader is used for the purpose of digging a hill-shaped earth mass and loading the thus digged earth on a dump truck. FIG. 2 illustrates a series of operation patterns ranging from digging by the wheel loader to loading of digged earth on a truck, and this is a most frequently employed operation pattern which is called V-shaped operation.
To shorten a working time in this V-shaped operation (comprising operations .circle.1 to .circle.5 as represented by arrow marks), it is required that operations are performed in the following manner.
Namely, the wheel loader 1 moves forwardly toward the hill-shaped earth mass 2 at a second forward movement speed (F 2) (operation .circle.1 ) and when it moves close to the hill-shaped earcth mass 2 (by a distance in the range of 0.5 m to 1.0 m), it is caused to plunge into the hill-shaped earth mass 2 at a first forward movement speed (F 1) in order to enhance a trailing force when digging work is conducted (operation .circle.2 ). On completion of from the position of digging work at a second rearward movement speed (R 2) (operation .circle.3 ) and subsequently the direction of movement of the wheel loader 1 is changed so that it moves toward a truck 3 at a high speed of F 2 (operation .circle.4 ). When loading work of digged earth on the truck 3 is completed, the wheel loader 1 moves away from the truck 3 at a high speed of R 2 (operation .circle.5 ).
Heretofore, when the aforesaid operation .circle.1 is changed to the aforesaid operation .circle.2 , a speed stage should be shifted from F 2 to F 1 and moreover the bucket BK should come in contact with the ground, as shown in FIG. 4. However, this handling is very troublesome.
Namely, due to the fact that a boom lever 4 is grasped by an operator's right hand and a steering handle 5 is grasped by his left hand as shown in FIG. 3, the left hand should be parted away from the handle 5 at a time forward movement when a speed stage is shifted from F2 to F1. However, this handling is dangeous and troublesome. Since this handling is troublesome in this way, sometimes an execution of shift-down is forgotten or shift-down is executed after the wheel loader plunges into the hill-shaped earth mass, because he pays much attension to contact of the bucket with the ground.
When an execution of shift-down is forgotten, smooth scooping can not be effected due to reduced tractive force and thereby a sufficient amount of digged earth can not be scooped by the bucket. Further, when shift-down is executed with delay, a time for which driving force is not temporalily exerted is developed with the result that a body of the wheel loader is pushed back, an operator feels unpleasant or smooth scooping can not be carried out.
The present invention has been made with the foregoing background in mind and its object resides in providing an automatic speed changing apparatus for a wheel loader which assures that shift-down is automatically executed during digging work and handling of an operator can be reduced.